Previous methods used to fabricate fused optical fiber couplers relied upon manual techniques and equipment which were controlled by human operators. As a consequence, the coupler fabrication process was subject to errors, a lack of reproducibility, and excessive cost. These factors resulted in an inherent low production yield for couplers having an acceptable level of performance. Placing a human operator in the fabrication process, while suitable for some laboratory experiments, precluded the creation of efficient commercially marketable couplers since the labor intensive cut and try procedures were not cost effective. Furthermore, the fabrication process was more of an "art" which depended on the unique skills of a particular operator.
Typical of the state-of-the-art couplers are shown by Matt McLandrich in his U.S. Pat. Nos. 4,377,403 and 4,557,553. The first of these patents concerns a method of fabricating a fused single-mode fiber bidirectional coupler having fibers with claddings etched away that are twisted and fused together. Evanescent coupling is relied upon for information transfer. Evanescent coupling, a term well established in the art, is set forth by N. S. Kapany and J. J. Burke in their text entitled Optical Waveguides Academic Press (1972). The wavelength selective coupler of the second patent permits a wavelength selective coupling of data between a pair of fibers in a bidirectional system. Both of these patents concern the end product, the couplers and do not disclose the means by which reproducible couplers are made. A reference to Barnoski et al. in U.S. Pat. No. 4,054,366 concerns a laser fused coupling of multi-mode fibers that has its coupling dependent on a fused cladding-on-cladding interconnection. A patent to Kent A. Murphy, U.S. Pat. No. 4,426,215 concerns a method of fabrication a low loss fused biconical taper fiber optic coupler that works as a reflection and transmission star coupler. The technique disclosed calls for a twisting, pulling and pushing of fibers. No computer control or automation is referred to so that the construction appears to be operator intense. The low loss access coupler for multimode optical fiber systems of Bryon S. Kawasaki et al in their U.S. Pat. No. 4,291,940 relies on fusing twisted fibers to enhance mode mixing. The method of fabrication again appears to be operator intense and the reproducibility might not be as exact as needed for mass production techniques. Terry Bricheno et al's single-mode fiber directional coupler manufacturer of U.S. Pat. No. 4,591,372 traverses fibers longitudinally through a flame while stretching them between a pair of carriages going the same direction at slightly different speeds. The Bricheno et al disclosure states that there is an observation in the manufacturing process to monitor the amount of light coming through the fibers being coupled, although it does not elaborate further on how this observation is meaningfully brought into the control of the manufacturing process. The single-mode fiber coupler of U.S. Pat. No. 4,612,028 preserves polarization in the coupler and has etched twisted fibers heated while under axial tension. A desired coupling is measured by detectors located at ends of the fibers, so that the application of axial tension and heating could be halted by means unknown, presumably a human operator. The modal-insensitive biconical taper coupler of Tremblay et al in their U.S. Pat. No. 4,586,784 has a fused together tapered coupling between fibers to allow selective coupling ratios. This patent does not disclose any means of automatically making the coupler. It must be presumed to be labor instensive, absent any indication to the contrary.
From the foregoing, it is apparent that the coupling of an optical beam from one or more fibers to another fiber or fibers is well-known in the art. However, all the couplers seem to be fabricated in accordance with methods suitable for the laboratory with considerable human intervention. They are not by any inovative, reproducible technique having cost effectiveness for mass production.
Thus, a continuing need exists in the state-of-the-art for a method and apparatus for providing a precision and reproducible fabrication of optical fiber couplers using a computer to automatically control the fabrication steps and so be adaptable for commercial use.